A group of workers at a warehouse that supplies Walmart stores have gone on strike to protest what they say are dangerous labour conditions and retaliation by management against employees who complain about them.

Organisers for the activist group Warehouse Workers United, which is working in the booming warehouse industry that has grown up in the Inland Empire region of southern California, say at least 20 workers had walked off the job and were protesting outside the gates of a warehouse run by transport firm NFI.

Labour abuses in the Inland Empire's warehouse industry have recently been highlighted in various media and academic reports. Experts say the industry, which is thought to employ some 110,000 people, is often staffed by poor, immigrant workers who toil for long hours with little pay and in unsafe conditions. One recent survey by the WWU and the University of California interviewed 101 workers and found that 83 of them said they had suffered a job-related illness.

The striking workers at the NFI facility have filed an official complaint with California labour authorities on conditions that they say show inadequate safety protection, not enough access to drinking water in warehouses that can reach 125F on a hot day, and a management culture that bullies them.

They also say that workers listed in their legal complaint have since been retaliated against by things such as demotions or reduced hours, and the threat of losing their job.

One of the strikers, David Garcia, 29, said he had walked out of work in order to fight for better conditions. "We were told we would lose our jobs if we did not shut up," he said.

The workers are not members of a union and risk dismissal by refusing to work. "I am willing to risk it for the rights of myself and my co-workers," said Garcia.

The strike comes as WWU begins a six-day "pilgrimage" of warehouse workers who will walk 50 miles from the high desert of the Inland Empire to downtown Los Angeles.

The march will aim to use the route taken by the shipping containers that arrive by boat in LA and are then driven to the vast warehouse hub of the Inland Empire, where they are unpacked and then reloaded for entry into the distribution networks of major retailers.

In its actions the WWU has specifically targeted Walmart warehouses due to the company's scale, which many experts say dictates the practises of the rest of the entire industry. "We are hoping Walmart will intervene. They must be aware of these problems," said Guadalupe Palma, a WWU director.

Neither NFI or Walmart responded to requests by the Guardian for comment.

Previously, NFI has disputed the specific claims of workers. It is contesting the official complaint. It says it does not retaliate against workers and adheres to safety standards. Walmart, meanwhile, has said that it expects all its the companies in its supply chain to conform to "the highest standards" and take corrective action if abuses occur.